A numerical control apparatus (hereinafter referred to as "NC") requires two kinds of cables to control one axis: one is a command cable which connects a servo amplifier to a servo control circuit and the other is a feedback cable which connects a pulse coder to the servo control circuit. Corresponding to these two kinds of cables are at least four interfaces (connectors) which must be installed.
FIG. 3 illustrates a connecting relationship between a servo control circuit for a conventional NC, a servo amplifier, and a servo motor. As shown in this figure, a command cable 87a is installed between the servo control circuit 11a for NC 1a and the servo amplifier 2a via interfaces 81a and 82a. Also, a feedback cable 88a is installed between the servo control circuit 11a and a pulse coder 61a, which detects the position of servo motor 6a, via interfaces 83a and 84a. When the pulse coder 61a is of a position data hold type, it is necessary to install a battery cable 90a between a battery 91, which supplies power to the pulse coder 6la, and the servo control circuit 11a via an interface 85a. Between the servo motor 6a and the servo amplifier 2a, a power transmission cable 96 is installed via interfaces 94 and 95.
As described above, three cables 87a, 88a, and 90a are required around the NC 1a to connect the uniaxial servo motor 6a to the servo control circuit 11a. Accordingly, many interfaces such as 81a, etc. are required.
However, the number of controlled axes to be controlled by one control section (numerical control apparatus) tends to increase year by year because of the advances in numeric control and the demand of the market. As the number of controlled axes increases, the number of interfaces (connectors) with the servo control circuit increases, resulting in an increase in the proportion of connectors in the mounting space of the control section.
On the other hand, there is a strong market demand for miniaturizing numerical control apparatuses. Although the integration of control circuits has been achieved to meet this demand, a considerable number of connectors must be mounted. Accordingly, miniaturization cannot be attained effectively as a whole.
Further, because cables are concentrated on the numerical control apparatus, it is hard to perform wiring work around the numerical control apparatus, thus increasing time required for wiring work.